Shape memory has been observed to occur in polymers and alloys. While, for some shape memory polymers (SMPs) and shape memory alloys (SMAs), the observed shape memory and shape recovery rely on the crystallization behavior of the constituting material, for other SMPs, it is triggered by the glass transition temperature. SMPs are usually capable of undergoing very high (up to a few hundreds percent) and/or variable deformations but somewhat incomplete recovery, whereas SMAs usually exhibit complete recovery of somewhat much smaller deformations (up to 8%).
Side-chain liquid crystalline elastomers (LCEs) have also been widely studied because of their thermomechanical properties that include large strain reversible actuation and soft elasticity. The thermally stimulated actuation behavior is explained by a coupling between liquid crystalline order and rubber elasticity resulting from the underlying crosslinked structure.
Main-chain liquid crystalline polymers (MC-LCPs) have been studied less frequently than side-chain systems, presumably due to, in part at least, some unfavorable properties, i.e., high-phase transition temperatures (from mesophase to isotropic phase) and low solubility. Studies conducted with a MC-LCP slightly crosslinked by a reaction with α,ω-hydride terminated siloxane showed that cross-linking does not disturb the liquid crystalline phases, while the crosslinked sample showed rubber-like elasticity (K. H. Hanus et al., Colloid Polymer Science, 1990, 268, 222).
Higher actuator performance had been expected for main-chain liquid crystalline elastomers (MC-LCEs) due to an enhanced coupling between their intrinsically high, yet labile, ordering of the mesogenic units and network strain as compared to their side-chain analogues (P. G. de Gennes, C. R. Acad. Sci. Ser. Iib: Mec., Phys., Astron. 1997, 324, 343). Demonstration of the synthesis of one such MC-LCE was recently shown (B. Donnio et al, Macromolecules, 2000, 33, 7724 and references therein), but the product was not thermomechanically characterized.
Two salient properties of LCEs, namely, the phase transition temperature for the liquid crystalline phase (from mesophase to isotropic phase upon heating and the opposite direction upon cooling) and the elasticity of the crosslinked polymer network, make LCEs candidates for shape memory materials. Challenges exist, however, for main-chain liquid crystalline polymers due to their high transition temperatures and processing difficulties.
Shape memory materials require not only tailored transition temperatures (including for LCEs, isotropization), but also specific mechanical properties. To date, the syntheses of LCEs exhibiting capabilities as shape memory elastomers possessing low transition temperatures have not been achieved.